1. Field of the Invention
The present invention relates to the field of devices and methods for assisting in the perception of sound for the hearing impaired and more specifically to a transducer type for listening to sounds by an abutment to the head for the transmission of transducer vibration to the skull structure. More particularly, the present invention relates to a bone conduction hearing aid having the vibrator element directly in contact with the skin surface of the patient's head.
2. Description of the Prior Art
The human auditory system, consisting of the ears and associated brain structures, possesses remarkable signal processing capabilities. We hear sounds from those that are barely detectable to those that reach the threshold of pain—a difference of about 130 decibels or a ratio of about 10 trillion to one. In addition, the auditory system is a powerful sound analyzer. Rapid changes in the frequency and amplitude of sounds over time, such as those in human speech, are readily detected and decoded. Indeed, human communication is made possible not only because of our special ability to produce speech, but also because of our capabilities in auditory signal processing.
The perception of sound is achieved in human beings through the ear. Sound is transmitted to the ear through vibrations in the air which is known as air conduction. However, it can also be transmitted through the human bone structure (the skull). A very instant example is the ability of a person to perceive the sound from his chewing even when the ears are blocked. This form of sound transmission is termed as “bone conduction”.
In normal hearing, sound passes along our ear canals to the eardrum causing the surface of the eardrum to vibrate. These vibrations are passed to the ossicles by a process called air conduction. In turn, these vibrations pass acoustic energy across the oval window and innervate the movement of the cochlear fluids. Movement in this fluid bends the hair cells along the length of the cochlea, generating signals in the auditory nerve. These signals are then transferred to the brain, thus the interpretation of sound.
Like most natural processes of the body, the ability to hear is made possible by an intricate process involving many steps. The mechanical portion of this intricate process takes place in the outer ear, middle ear, and the inner ear. The outer ear, the auricle, collects sound waves and leads these waves into the middle ear. The middle ear couples the sound waves in the air-filled ear canal to fluid of the inner-ear (perilymph). The middle ear, containing the eardrum (tympanic membrane) and three tiny bones (malleus, incus and stapes), is an interface between the low impedance of air and high impedance of inner ear fluid. Pressure induced vibrations of the tympanic membrane ultimately induce a proportional motion of the stapes, the smallest of the three auditory ossicles in the middle ear. This motion is the output of the middle-ear. The stapes transmits this motion to the inner ear. In the inner ear, this motion produces a large pressure in the scala vestibule, a perilymphatic channel on one side of the cochlear duct, in comparison with the scala tympani, a perilymphatic channel on the other side of the cochlear duct separated from the tympanic cavity by the round window membrane. The pressure difference between the two scalae in turn causes a traveling wave to move apically on the basilar membrane. The motion of the basilar membrane causes the cilium of receptor cells, also known as the inner hair cells (IHC) to move, which in turn causes firing of the auditory nerve. This process produces the sensation of hearing.
The ability to hear and the sensitivity at which one is able to hear is diminished by two basic types of ear pathologies that are commonly referred to as i) conductive hearing loss, and ii) sensory-neural hearing loss. Conductive hearing loss may be traced to either a pathological condition of the middle ear or the middle-ear cavity, or impairment (i.e., blockage) of canal or the outer ear. This type of hearing loss is routinely repaired by otology surgeons. On the other hand sensory-neural hearing loss is due to a pathological condition of the inner ear and is nearly impossible to repair via surgery. Just in the United States, it is estimated that over 26 million people suffer from some type of hearing loss problems.
Loss of auditory function is commonly associated with reduced power to detect and decode speech. Persons who experience significant hearing loss are likely to become isolated from normal verbal exchanges. They then lose out on nuances of speech that are vital to that most important and distinctive human trait—communication. As a result, additional problems can develop as a result of misunderstandings and incomplete receipt of information experienced by the person with hearing loss.
Assessment of hearing loss is normally conducted by testing for minimum sound amplitude levels that can be detected. There are two forms of tests used for the basic evaluation of auditory function. The first, air-conduction testing, involves presenting precisely calibrated sounds to the ears, usually by routing the signals through headphones to the external ear canal. The second, bone-conduction testing, sends precisely calibrated signals through the bones of the skull to the inner ear system. Stimulation is received at the skull by placing a transducer either on the mastoid region behind the ear to be tested or through transducer placement on the forehead.
Differences between hearing loss profiles for air and bone conduction can indicate a probable locus for a hearing problem. For example, if air-conduction scores are poorer than bone-conduction scores the indication is that a flaw is present in the mechanisms that carry sound from the eardrum to the inner ear. Remediation of this type of problem might involve surgical repair of damaged conductive elements. If bone-conduction and air-conduction scores show similar levels of hearing loss, then it is likely that there is a deficiency in sensory-neural function. This variety of hearing loss can result from illness, sustained exposure to loud sounds, drug effects or an ageing hearing system. Frequently, with sensory-neural losses it is possible to improve a person's hearing with modern digital hearing instruments.
People with hearing problems also have to resort to hearing aids that are principally used external to the ear. Conventional hearing aids make sounds louder and deliver the acoustic energy to the ear canal via an ear-mold. The ear-mold fits snuggly in the aperture of the ear canal, thus creating a hermetic seal, which only permits sound coming out of the aid to enter the ear. These amplified sounds are then heard through the ear canal via normal air conduction. Sometimes amplification through air conduction does not provide enough amplification to innervate the cochlear fluids. In cases like these where air conduction does not serve the purpose, amplification via bone conduction is the next option.
Hearing by bone conduction as a phenomenon, i.e., hearing sensitivity to vibrations induced directly or via skin or teeth to the skull bone, has been known since the 19th century. The interest in bone conduction was initially based on its usefulness as a diagnostic tool. In particular, it is used in hearing threshold testing to determine the sensory-neural hearing loss or, indirectly, to determine the degree of conduction hearing loss by noting the difference between the air and the bone thresholds.
The first electronic bone conduction device was built in 1923 but it was too bulky for any practical purpose. In the past two decades, significant improvements have been made in the development of bone oscillators. With proper power supply instrumentation, these Bone Oscillators permit transduction of low and mid range frequencies.
In the hearing threshold testing field, which is one of the relevant application areas of interest of this patent, one of the most commonly used bone conduction transducers is the Radio Ear B-71 type, which is introduced here as a part of the relevant prior state of the art. The B-71 transducer is an electromagnetic-type transducer of the variable reluctance type. Variable reluctance type transducers function according to the horseshoe magnet principle where there is a small air gap between the armature (basically the permanent magnet) and the yoke. By superimposing a signal magnetic flux (generated by a coil whose dimensions are not so critical) the force in the air gap, between the yoke and the armature, will vary accordingly. This force can be used to generate vibrations in the transducer.
The B-71 transducer has a plastic housing with a 1.75 cm2 circular attachment surface toward the head, as illustrated in FIG. 1. With a steel-spring headband, the transducer is pressed with a total force of approximately 5-6 Newton against the mastoid area behind the ear. Internally, as briefly pointed out above, the transducer consist of an armature, a yoke, and a small but essential air gap which disrupts the magnetic flux path. The magnetic flux is composed of the static flux generated by the permanent magnet and the dynamic flux generated by the current in two coils. The total weight of the B-71 transducer is 19.9 g.
Some drawbacks of the currently available variable reluctance type transducers can be pointed out. The first drawback is related to the intrinsic design and number of components involved in the design of this type of bone conduction transducers, as shown in FIG. 1. It is well know by audiologists the problems involved with this type of bone-conduction vibrators and the continuous necessity of constant recalibration of this type of actuators due to accidental dropping or simply loss of calibration during normal use. During the calibration process, screws have to be re-adjusted to obtain the expected frequency response from the transducer.
A second drawback is related to the poor frequency response of this type of actuators which in the midrange frequencies and above 4 kHz deteriorates sharply. FIG. 2 provides the frequency response for the B-71 transducer when driven under constant input amplitude for all the frequencies considered. Specifically, in FIG. 2 the amplitude of the input sinusoidal waveform was taken as 100 mV. As it can be seen, the frequency response of the B-71 actuator is very poor over the frequency range considered (200 Hz to 10 kHz) and becomes drastically low above 4 kHz. This situation has limited the bone conduction devices in the market to operate only up to 4 kHz. Ideally, a Bone Oscillator device with a flat frequency response (not more than ±5 dB) up to 4 kHz and if possible, above 4 kHz would be required.
This poor frequency response of the current state-of-the art technology has forced the current hearing threshold testing field standards to be adapted to this situation and the limitation in the state of the art of this technology. Table 1 shows the current ANSI S3.43 (1992) standard requirements for bone conduction transducers. Improvements in the 10 existing bone-conduction transducer technology will significantly benefit the possibility of considering a more realistic standard for bone conduction hearing threshold testing.
TABLE 1ANSI S3.43 StandardBone Conduction Oscillator ANSI S3.43 (1992)HL SettingRMS Force Levels (dB re: 1 Dn) 250 Hz25 dB72.0 500 Hz40 dB78.0 750 Hz40 dB68.51000 Hz40 dB62.51500 Hz40 dB56.52000 Hz40 dB51.03000 Hz40 dB50.04000 Hz40 dB55.5
A third drawback of the currently available type of bone-conduction oscillators is the necessity of being operated by an amplifier (so called audiometer) that needs to be specifically calibrated so that the bone-conduction oscillator provides the expected output performance. In the calibration process, the audiometer output voltage is adjusted for each frequency step required: 250 Hz, 500 Hz, 750 Hz, 1000 Hz, 1500 Hz, 2000 Hz, 3000 Hz and 4000 Hz. For each of these specific frequencies, the audiometer is tuned so that the bone conduction oscillator will provide the output force value required by the ANSI standard. This is of course not only time consuming but extremely limiting if the bone conduction device is expected to be used in a different frequency point from those calibrated. Further, it is not possible to use this type of transducers to perform a test involving a continuous frequency sweeping.
Another drawback of conventional bone conduction hearing devices is the use of a magnetic transducer, which creates electromagnetic interference (EMI). This EMI interferes with surrounding medical or radio frequency devices.
Thus, there has been a long-standing problem inherent in the construction and function of conventional bone conduction transducers used in hearing aids and for auditory testing. Typically, these devices have been restricted in the usable frequency range, particularly above 4000 Hz and they have been limited in the amplitude with which sound can be presented to the skull. Bone conduction transducers have relied on electro-mechanical components to propagate vibrations. In every day use, it has been repeatedly observed that such transducers do not operate in a linear manner. As a result, individual audiometers must be calibrated to the idiosyncratic properties of the bone conduction transducer to be used with that system. A further problem arises when the old style transducers are used on a daily basis. When dropped, the transducers frequently break or alter their output characteristics.
The previous drawbacks show the necessity of improving the existing state of the art on bone conduction transducers. Therefore there exists a necessity to provide an actuator with the correct physical size, and with a desired frequency range from 100 to 8000 Hz, linear operation across the relevant range, significant increases in power levels and in a rugged package.